A technique for straightening curly or frizzy hair can be classified according to a hair straightening agent to be employed. When a keratin reducing substance is used as a hair straightening agent for permanent straightening, the hair is straightened by applying the keratin reducing substance to the hair to cleave disulfide bonds therein, stretching the curly hair by combing and then applying an oxidizing substance to the hair to re-establish disulfide bonds. On the other hand, a hair straightener using a highly concentrated hydroxide (alkali agent) straightens the hair by converting disulfide bonds in the hair to lanthionine bonds by means of hydroxide ions, removing the remaining alkali by rinsing with water and shampooing, and then neutralizing the hair.
Such a chemical change in the hair is caused by adjusting pH to about 9 in the case of a permanent hair-straightening composition and to 12 to 14 in the case of a hair straightener. It is however known that treatment of the hair under such high pH conditions has a considerable adverse effect on the hair or scalp treated. The hair damage (generation of split or cut hair) is particularly marked when the hair is treated with a hair straightener.